Bleaching of cellulose pulp in towers in completely filled and closed system



April 11, 1967 s. o. RYDIN 3,313,678

BLEACHING OF CELLULOSE PULP IN TOWERS IN COMPLETELY FILLED AND CLOSED SYSTEM Filed Feb. 13, 1963 2 Sheets-Sheet 1 FIG] 11 April 11, 1967 o RYDINV 3,313,678

S. BLEACHING OF CELLULOSE PULP IN TOWERS IN COMPLETELY FILLED AND CLOSED SYSTEM Filed Feb. 15, 1963 2 Sheets-Sheet 2 FIG.2 9

1s 1s v United States Patent Ofifice 3,313,678 Patented Apr. 11, 1967 3,313,678 BLEACIWG F CELLULOSE PULP 1N TOW- ERS IN CDWLETELY FILLED AND CLOSED SYSTEM Sten 0. Rydin, Gstrand, Timra, near Suntlsvall, Sweden, assignor to Svenska Cellulose Aktiebolaget, Sundsvall, Sweden Filed Feb. 13, 163, Ser. No. 258,322 Qlaims priority, application Sweden, Feb. 14, 1962, 1,690/62 6 Claims. (Cl. 162-19) The present invention relates to continuous treatment, substantially bleaching, of cellulouse pulp in towers. Towers used for this purpose are constructed, on the one hand, for operation with upward flow of the stock and, on the other hand, for operation with downward flow of the stock. Distinction is made between towers, operating with low stock consistencies in the range of l-%, medium high consistencies in the range of 58% and high consistencies in the range of 820%. The invention has ref erence to towers, in which the contents are caused to flow either upwards or downwards, and in both cases they operate with stock of medium high and high consistencies.

In the art of bleaching cellulose stock the development has proceeded from batch bleaching in open tanks to continuous bleaching in reaction towers, so-called bleaching towers. The transition to continuous bleaching in towers hasbeen resorted to for the purpose of obtaining better pulp qualities at lower costs. For economical reasons it is desired to effect the bleaching at so high a stock consistency as possible, on the one hand for the purpose of saving tower volume and, on the other hand, in view of the fact that the contents of the tower often will have to be heated, in which case stock of a high consistency will require a lower consumption of heat than stock of a low consistency.

The early towers adapted for high stock consistencies were of a construction in which the stock was caused to move from the top end portion of the tower towards the bottom end portion thereof. Thickened stock, delivered, for example, from a rotary thickener, was supplied at the top end portion of the tower and at the bottom end portion of the tower there was flushing means, diluted stock being discharged through a duct at the bottom end portion of the tower and supplied to a centrifugal pump and hence to a washer. Towers of this type either were open at the top or also they were covered with a roof, so that a space filled with air, or other gas, was formed above the stock. An example of a tower, open at its top, is disclosed in the Swedish patent specification No. 140,977.

In that type of towers there is, among other things, the difiiculty of maintaining a discharge of pulp from the tower that will, at any moment, balance the quantity of stock being supplied to the tower. Hence, in industrial operation, the level of stock within the tower will sometimes rise and sometimes sink and, due to this, variations in the bleaching time in the tower will occur and further supply of chemicals in a subsequent bleaching stage, if any, will be rendered dificult, because of the fact that it is not possible to maintain, at every moment, a dosage of chemicals strictly corresponding to the quantity of pulp discharged from the tower, unless there be provided very complicated controlling means. This will cause variations in the quality of the bleached pulp. A further drawback inherent in towers-in which there is formed, above the stock, a space filled with air, or other gas-is the problem of satisfactorily sealing the tower, for in case gaseous bleaching agents such as chlorine or chlorine dioxide are used, which, it is true, usually are supplied in aqueous solution but which, under the circumstances, give off some gas to said airfilled space, then that strongly corroding damp gas will be obtained within the top end portion of the tower and in view hereof the tower must be carefully sealed so as to avoid, because of escaping gas, losses of bleaching agent and corrosion of surrounding apparatus and also inconveniences to the people in the room.

A second fundamental type of bleaching towers that has been developed, is one in which the stock is caused to flow from below and upwards. In building towers of this kind assistance was rendered by the development of pumps, so that at present it is possible to pump stock having a consistency of up to 15 to 20%. When bleaching in towers of this kind it is customary either to remove the pulp at the top end portion of the tower by scraping it out into a collecting channel, concentrically surrounding the top end portion of the tower, and diluting the pulp in said channel with liquid to a consistency in which it can be pumped by means of a centrifugal pump, for example, in the way shown in FIGURES 1 and 2 of the Swedish patent specification No. 142,609, or also to dilute the pulp at the top end portion of the tower to the extent that it will liquefy and flow over the top edge of the tower and down into said channel. Also, there exists a combination of said two ways. When operating with a medium high and a high stock consistency also towers are used which, accordinto what has been said above, are open at their tops or covered with a roof. The reason is that it has not been possible to discharge the pulp in any other way than by scraping it off or by flushing it away from a tower, which is open at its top.

Such way of discharging the pulp from the tower involves, inter alia, the drawback of bubbles of air, or of other gas, becoming intermixed with the pulp while the latter is being diluted in the channel or at the top end portion of the tower, which will, in turn, impair the capacity and the washing by the filter g device on which the pulp, customarily, is dewatered and washed after the treatment in the tower. The intermixin-g of air occurs because of the relatively great differences between the speed of the stock and the speed of the diluting liquid which exist in the channel surrounding the top of the tower, when the pulp flows down into the channel or when a jet of diluting liquid is introduced into the top end portion of the tower. The products dissolved from the stock during the bleaching reaction, often form, together with water, heavily foaming liquids and, furthermore, the fibers of the stock preventing gas bubbles from effectively ascending to the surface of the stock and also from escaping there, the foam and the intermixing of gas, respectively, will be of a rather permanent duration. Often the formation of foam will become so heavy that it will prevent effective supervision or control of the level of pulp in the channel, which in serious cases may result in the channel becoming emptied by suction, air being then sucked into the discharge duct of the channel and making things still worse, particularly if a centrifugal pump is connected to the duct following the channel. In that case the operation in the washers might become completely spoiled.

In towers of the kind in which the pulp is diluted at the top end portion of the tower to such an extent that it will flow over the edge of the tower and down into the surrounding collecting channel, the dilution must be carried out to a low consistency, usually of the order 1 /z-3 If in that case the tower is built for operation with stock of a relatively high consistency, say 820%, the diluting liqaid will penetrate deep down into the tower by which the volume of the tower cannot be utilized in an effective way.

The present invention relates to a method by which the aforementioned drawbacks will be overcome, the advantages connected with high-density bleaching being maintained at the same time. The invention is based upon the use of entirely closed bleaching towers and upon always keeping the towers filled, or substantially filled, with stock and also upon operating with high stock consistencies. Closed bleaching towers, operating while completely filled, previously have been used in connection with low stock consistencies, such as 25%, but then the low stock consistencies necessitate the use of large tower volumes and also result in the heat consumptionarising because of the necessity of raising the temperature of the stock in ceratin towers up to, for instance, 4080 C.becoming very great. Such towers are used mainly in conneetion with the chlorination of stock, and chlorination, as a rule, is carried out at low stock consistencies, such as from 25 for instance at 3.5%

The possibility of operating with an entirely closed and permanently filled tower at high stock consistencies and with a downward flow of the stock within the tower is based on the provision of a counterpressure at the stock discharge end of the tower, said counterpressure balancing the column of stock within the tower and preventing the formation of a vacuum at the top end portion of the tower.

The possibility of operating with an entirely closed and permanently filled tower at high stock consistencies and with an upward how of the stock within the tower is based on the step of diluting that part of the stock which is at the top end portion of the tower, prior to said part of stock leaving the tower, the dilution being carried on to a consistency at which the stock becomes fluid but not so far that the liquid will, to any essential extent, penetrate into the underlying layer of stock. The term fluid as used in the present connecton means that the stock at the top end portion of the tower becomes so plastic that it can be pressed upwardly to the discharge opening of the tower and out through said opening. This will occur when the dilution is carried on to a consistency of about 6 to 7% but it is possible, without inconvenience, to dilute the stock, to a consistency of about without water penetrating, in any essential degree, into the underlying layer of stock. The plasticizing of the stock in connection with the dilution thereof at the top end portion of the tower may be facilitated by the provision of stirring members at the top end portion of the tower. This is particularly the case in towers having large diameters.

Two ways of application of the method of supplying stock of a consistency of more than 7%, preferentially 9-18%, say -15%, will appear from the following description, reference being had therein to the accompanying drawings.

FIGURE 1 shows a bleaching tower, in which cellulose stock of a high consistency is caused to move from the top of the tower to the bottom thereof. The stock, which for example is of a consistency of about 14% is supplied to a thick stock pump 1, which through a pipe line 2 pumps stock into the top end portion of the tower 3, said tower being provided with an air exhaust valve (not shown). On top of the tower there is provided combined means 4 for distributing stock within the tower and for intermixing chemicals with the stock, said means being connected to a pipe line 13 through which the chemicals are supplied. At the bottom end portion of the tower there is provided means, of a known type, for flushing away stock, said means, in the embodiment shown, comprising a number of flushing spouts 5 provided around the lower part of the tower and positioned some distance above the bottom of the tower. At the bottom end portion of the tower there is a recess 6, above which there is provided a stirring propeller 7 and an inlet 8 for liquid. By supplying diluting liquid through the spouts 5 and the inlet 8 the stock at the bottom of the tower becomes diluted and is kept in motion by means of the propeller 7 so that around the latter a certain volume of stock, of a uniform concentration, is kept in motion. This diluted volume of stock is led away through an outlet opening 9 to a discharge duct 10. The consistency may now, at will, be between about 1 and 5%, for example between 2 to 4% The discharge duct 10 leads up to a washer 11, which is positioned at about the same level as, or somewhat higher than, the top of the tower. At 12 provision is made for adding more diluting water to the stock, if desired, so that the stock will be brought to a consistency, which is suitable for the operation of the washer. The discharge duct 10 extends up to such a high level that at the outlet 9 of the tower such a strong counterpressure is formed that no vacuum can arise within the top end portion of the tower. Hereby it is attained that the tower will always be filled with stock as long as a sufficient amount of stock is being supplied at the top end portion of the tower. The stock feeding pump 1 continuously supplying the tower with stock, a uniform downward flow of stock will be obtained within the tower. Consequently, the above described mode of operation makes it possible always to maintain the same volume of stock within the tower, and furthermore that the tower will always deliver the same amount of pulp as the amount of stock supplied through the pump 1 and, finally, that the stock will thereby always stay within the tower for the same length of time. In this 'wa'y the previously mentioned drawbacks of varying bleaching times are elimi nated and, furthermore, there is obtained the advantage of not having to make the tower any larger than what will answer the most preferred time of treatment and the desired consistency of the stock. At its top the tower is sealed by the stock within the pipe line 2, due to which gases cannot escape from the tower. A further advantage is that the pump 1, feeding the stock into the tower, also is able to cause the stock to move on from the tower to the wash 1]., which is at the same level as, or higher up than, the top of the tower. Consequently, the pump provides for the entire transportation of stock from the feeding-in thereof into the tower and all the way to the feeding-in of the stock at the intake end of the wash, due to which there will be no need of a lifting pump, arranged after the tower. I

FIGURE 2 shows a bleaching tower, in which cellulose stock of a high consistency, for example, about 14% is to be fed from the bottom of the tower and upwards within the same. The thick stock pump 1 feeds the stock, which has been thickened by means of a filtering device or some other suitable apparatus, through the pipe line 2 to the; tower 3, which is provided with an air exhaust valve (not shown). At the bottom end portion of the tower there is provided combined means 4 for distributing the stock within the tower and for intermixing hrnicals with the stock. Chemicals, for example, hypochlor'ite', caustic soda, or chlorine dioxide water, are dosed through the pipe line 13 to the intermixing means 4. The stock being supplied to the pump 1 continuously, there is obtained a continuous upward movement of the stock within the tower 3. At its top end portion the tower tapers towards the discharge opening 9. Approximately at that point of the tower where the tower begins to become narrower there is provided means for diluting the stock by supplying diluting liquid in a manner known per se. In the em bodiment shown the supply of diluting liquid takes place along the periphery of the tower through tangentially die rected spouts 14 and stirring means 15 provided centrally within the top end portion of the tower, said stirring means being provided with spouts 16 for supplying diluting liquid at the center. The stock, diluted to a consistency of about 1-7%, for example 4-6%, leaves the tower through the discharge duct 10 by way of the discharge opening 9. In most cases the stock leaving the tower is to be conveyed to a wash and therefore has to be further diluted, which preferably is done within the discharge duct 10 by supplying diluting liquid from a pipe line 17. The stock, thus further diluted to a consistency of about 1 to 2% is then led to the filtering device (not shown). The hereinbefore described arrangement makes it possible to operate with an entirely closed and a completely filled system at high stock consistencies. Hereby it is possible to overcome the previously mentioned drawbacks, which are caused by the fact that air, or other gas, may be intermingled with the stock, while the latter is being diluted at the top end portion of the tower or in a collecting channel surrounding the top end portion of the tower. Consequently, it is possible to operate with a small tower volume, a effect in the following wash.

Having now described my invention, what I claim as new and desire to secure by Letters Patent is:

1. In a process for continuously bleaching cellulose stock in a reaction tower by first treating the stock with a bleaching agent and afterwards filtering the bleached stock discharged from the tower, the improved technique which consists in keeping said reaction tower completely filled with stock, bleaching at a consistency greater than 7%, and diluting the stock to a consistency of 17% immediately before discharging the same from the tower, and conveying the diluted stock from the tower to a filtering device in a continuous column, all steps being performed in a closed system wherein bleaching agents are confined to the system and air is excluded.

2. A process according to claim 1, in which the stock is moved downwardly in the tower and is diluted to a consistency of 24% immediately before being discharged from the tower.

3. A process according to claim 1, in which the stock is moved upwardly in the tower and is diluted to a con good heat economy and a good sistency of 4-6% immediately before being discharged from the tower.

d. A process according to claim 1, wherein the stock is caused to travel in a downward direction through the tower while being subjected to the action of the bleaching agent, and a hydrostatic pressure balancing the column of stock within the tower is maintained by the column.

5. Process according to claim 1 in which the stock is caused to travel through the main part of the tower at a consistency of 9l8%.

6. A process according to claim 1, wherein said filtering is carried out in a filtering device at a level which is at least approximately the same as that of the top of the tower.

References Cited by the Examiner UNITED STATES PATENTS 1,643,566 9/1927 Thorne 162-19 2,089,992 9/1937 Campbell 162-237 2,474,862 7/1949 Richter l62237 2,662,821 12/1953 Muench 162237 2,711,359 6/1955 Johansen 162-60 2,769,710 11/1956 Becker 162-17 2,876,098 3/ 1959 Schandroch 162-237 3,007,839 11/1961 Richter 16260 DONALL H. SYIVESTER, Primary Examiner. MORRIS O. WOLK, Examiner. HOWARD R. CAINE, Assistant Examiner. 

1. IN A PROCESS FOR CONTINUOUSLY BLEACHING CELLULOSE STOCK IN A REACTION TOWER BY FIRST TREATING THE STOCK WITH A BLEACHING AGENT AND AFTERWARDS FILTERING THE BLEACHED STOCK DISCHARGED FROMTHE TOWER, THE IMPROVED TECHNIQUE WHICH CONSISTS IN KEEPING SAID REACTION TOWER COMPLETELY FILLED WITH STOCK, BLEACHING AT A CONSISTENCY GREATER THAN 7%, AND DILUTING THE STOCK TO A CONSISTENCY OF 1-7% IMMEDIATELY BEFORE DISCHARGING THE SAME FROM THE TOWER, AND CONVEYING THE DILUTED STOCK FROMTHE TOWER TO A FILTERING DEVICE IN A CONTINUOUS COLUMN, ALL STEPS BEING PERFORMED IN A CLOSED SYSTEM WHEREIN BLEACHING AGENTS ARE CONFINED TO THE SYSTEM AND AIR IS EXCLUDED. 